Optical waveguides can be used to expand or replicate the exit pupil of an optical system in one or two dimensions. Typically, light from the exit pupil of the imaging system is received in the waveguide through an in-coupler, and travels through the waveguide in a direction, while some of the light exits a grating structure of the waveguide creating an expanded pupil. The remaining light that does not exit the grating structure is wasted and typically exits or is absorbed at the edges of the waveguide. Current waveguides typically use low-efficiency single pass grating structures to achieve pupil uniformity and field of view uniformity, which results in a large amount of wasted light.
One issue with current waveguides is angular field uniformity. Current systems attempt to achieve a uniform display intensity over the expanded exit pupil and a range of eye reliefs, also referred to as the eye box. The wasted light may result in nonuniform display energy which may be perceived, for example, by the user as either high angular-frequency banding in certain areas of an image displayed in the eye box or as variations in image brightness in different areas of the eye box.
Another issue with such current waveguides is efficiency. Because of the wasted light, devices that use waveguides may compensate by using higher power illumination systems. However, such systems require higher cost components, require large amounts of energy, generate excessive heat, and result in excess stray light.